Although many ceramic materials have been proposed as cutting tools for various applications, and some of these materials such as sintered or hot pressed silicon nitride are used quite successfully for some applications, aluminum oxide-titanium carbide is generally recognized as the best all around ceramic tool material. Commercial tools of this type are known to be produced by expensive and cumbersome hot pressing; see, by way of illustration, Ogawa et al, U.S. Pat. No. 3,580,708; Bergna et al, U.S. Pat. No. 3,542,529; and Ogawa et al, U.S. Pat. No. 4,063,908. In Brun, Lee and Szala, U.S. Pat. No. 4,515,746, a particulate mixture of powders of metal hydride, carbon and relatively inert ceramic powder, e.g., alumina are hot pressed to form useful composites comprising, for example, alumina-TiC when alumina, titanium hydride and carbon are hot pressed. Several recent disclosures, however, have shown that ceramic composites, e.g., alumina-TiC composites, can be sintered to a closed pore state, either by using specific oxides as sintering additives, e.g., Y.sub.2 O.sub.3 as in Kanemitsu et al, U.S. Pat. No. 4,356,272; Japanese Patent Publication 81,140,066 (Chem. Abs. 96:109112w) and Japanese Patent Publication No. 81,140,067 (Chem. Abs. 96:109111v); or by using titanium oxycarbide as in Japanese Patent Publication No. 79,103,407 (Chem. Abs. 92:63473b). Lee and Szala, U.S. Pat. Nos. 4,407,968 and 4,416,840 disclose sintering mixtures of aluminum oxide, carbon and elemental titanium or titanium hydride to composites having an Al.sub.2 O.sub.3 phase and a substoichiometric TiC phase. A very recent development is to use a significantly higher heating rate for the sintering process than is used in conventional practice; see Lee, Borom and Szala, U.S. Pat. No. 4,490,319.
If the heating rate of the sintering cycle is increased significantly over the current industrial practice, as disclosed in the above-mentioned U.S. Pat. No. 4,490,319, a dense, high quality ceramic article will be produced. Unfortunately, however, most commercial furnaces cannot produce high heating rates. Therefore, implementation of the new high-heating-rate process requires a sizable capital investment which decreases the economic incentives for adopting this new technology. On the other hand, addition of oxide additives in sufficient quantity to promote sintering of the ceramic powdered material to high density can alter the properties of the end products and diminish the usefulness of both the process and the product. Therefore, a method or methods which can produce materials without requiring major changes in facilities is still very much in need. The present invention solves such a need by providing new chemical compositions which can be sintered to a closed pore product with desirable properties using heating rates within the range of current industrial facilities. Moreover, if desired, the powdered ceramic mixtures provided by this invention can also be effectively densified using a rapid rate process, e g., that of the above-mentioned U.S. Pat. No. 4,490,319, to provide unexpected and desirable ultimate properties.
The following definitions are applicable to an understanding of this invention and/or the prior art:
SINTERING: development of strength and associated densification of a powder compact through the application of heat alone.
HOT PRESSING: the combined application of heat and of pressure applied through the action of a mechanical piston on the powder-filled cavity of a die. Under such conditions the pressure on the powder compact is non-uniformly applied due to die wall friction and the axial application of the piston force. Under proper conditions of temperature and pressure, densification of the compact can result.
HOT ISOSTATIC PRESSING (HIP): The simultaneous application of isostatic pressure and heat to a sample body whose porosity is to be reduced. Pressure is applied uniformly to the sample body by an inert gas. The sample body may be (a) a powder compact encapsulated in a gas impermeable, but deformable, envelope such as a tantalum foil can or a glass coating or (b) any solid substantially devoid of open porosity.
The sintered product of this invention is considered to be "substantially crystalline", because it is not atypical to encounter minor amounts of non-crystalline material (e.g. glasses) in the grain boundary phases.
This invention addresses a particularly troublesome problem encountered in the sintering of multiphase systems. Such systems frequently contain components, which will chemically interact at elevated temperatures and produce gases. If such chemical reaction proceeds fast enough to inhibit the desired densification or, if the nature of the reaction is such that it results in degradation of the system (i.e., undesirable solid, liquid or gaseous phases are produced), manufacture of the optimum product cannot be readily accomplished by sintering.
While not intending to be bound by any theory, it is believed that ceramic oxides, e.g., aluminum oxide, react with carbon or carbon-containing materials, e.g., titanium carbide, or the like, at temperatures exceeding about 1550.degree. C., emitting gaseous materials which in turn hinder the consolidation of the mixture on further heating. Such problems seem to intensify if free carbon is introduced with the carbide or if the particle size of the powdered component in the mixture is reduced.
It has now been discovered that if an additional component is included, such problems will be minimized. Specifically, according to this invention there will be included in the ceramic powder an additive which will become an effective scavenger of the evolving gas phase from the reaction between alumina, for example, and carbon, or a source of carbon. Judiciously selected such additives will also provide enhanced properties in the sintered products.
Typical examples of the invention are the addition of small quantities of either zirconium hydride or hafnium hydride to a ceramic powder mixture, e.g., a mixture of alumina and titanium carbide, and the like. The hydrides stay relatively clean during the conventional powder processing stages, but decompose to highly reactive components at about 1000.degree. C. This reactive metal forms oxides or carbides by reacting with the gaseous product evolving from the carbide-oxide or carbon-oxide reaction.
As a further advantage, small amounts of by-products, e.g., zirconium oxide or hafnium oxide, formed in the reaction, can also be retained in the high temperature phase to provide transformation toughening of the sintered product. In some cases, the resulting carbide from the process can dissolve into a carbide phase, for example, a titanium carbide phase, without detracting in any way from the desirable properties of the final product.
This invention is primarily described herein in respect to the Al.sub.2 O.sub.3 -TiC system, because this particular material system often presents the very problem in densification discussed herein above. However, the essential aspects of the sintering process disclosed herein are not dependent upon either the use of particular sintering additives, particular material proportions, or the nature of minor impurities. The process is expected to be broadly applicable to the sintering of powdered ceramic materials, that contain components which will chemically react at elevated temperatures to inhibit densification or degrade the system so that an undesirable sintered product results.